This invention relates generally to fluid catalytic cracking units, and more particularly to fluid catalytic cracking units having risers with improved hydrodynamics through the use of baffles.
In a fluid catalytic cracking (FCC) unit such as illustrated in FIG. 1, hydrocarbons are contacted in a reaction zone with a catalyst composed of finely divided particulate material. Inert diluent, such as steam, enters the riser and is mixed with catalyst. The hydrocarbon feed and an inert diluent, such as steam, are introduced to the riser 10 by a hydrocarbon feed distributor 5 which atomizes the hydrocarbon feed as it enters the riser 10. The hydrocarbon feed and inert diluent fluidize the catalyst and transport it in the riser 10. The catalyst promotes the cracking reaction. As the cracking reaction proceeds, a substantial amount of highly carbonaceous material, referred to as coke, is deposited on the catalyst. The coke-containing catalyst is separated from the hydrocarbon product in a separation zone 20 and removed from the reactor through conduit 30, while the hydrocarbon product exits through the top of the reactor. The coke is burned from the catalyst by contact with an oxygen-containing stream that serves as a fluidization medium in a high temperature regeneration zone 25. Coke-containing catalyst is replaced by essentially coke-free catalyst from the regeneration zone 25 through conduit 35. In some FCC units, there is a conduit 40 in which a portion of the catalyst is recycled without going through the regeneration zone 25.
FCC risers have traditionally suffered from vapor-catalyst slip caused by the inherent non-uniformities of upward moving particle-containing flows. These non-uniformities manifest themselves primarily as core-annular structures: the core of the flow is dilute and moves upward at a higher velocity, while there is a high concentration of catalyst near the wall which forms a dense, slow-moving annulus. The annulus can actually move downward in some cases. This annular flow results in decreased conversion in the riser because the faster moving dilute core under-converts the feed and the slower moving and/or downward moving annulus over-cracks the primary FCC products, leading to increased dry gas production.